X-Ray Observations of the Boundary Layer in Dwarf Novae at Low Accretion Rates

We present a spectral analysis of XMM-Newton data of 10 dwarf novae, 9 of which were observed during a quiescent state. The X-ray spectra indicate the presence of a hot, optically thin plasma with a temperature distribution consistent with a simple, isobaric cooling flow. The likely origin of the X-ray emission is cooling plasma in the boundary layer settling onto the white dwarf. Using a cooling flow model, we derive the temperatures, accretion rates, rotational velocities, and elemental abundances of the X-ray-emitting gas. We discuss the implications of our findings for the structure of the boundary layer. A comparison of X-ray and ultraviolet luminosities finds no evidence of underluminous boundary layers in any of the quiescent dwarf novae. The X-ray spectrum of EI UMa differs significantly from those of the other objects, showing a strong fluorescent Fe Kα line and a strong O VII line triplet. Based on the observational evidence, we argue that EI UMa is most likely an intermediate polar and not, as previously thought, a dwarf nova.